1. Field of the Invention
This invention relates to a convergence unit for an in-line color cathode ray tube.
In an in-line color cathode ray tube, the "red", "green" and "blue" electron beams lie in a common plane as they travel down the neck of the cathode ray tube to the deflection yoke. For television, it is found that the so-called self-converging yokes are adequate to preserve color convergence at the CRT screen. However with high precision CRT's intended to display data, misconvergence of the CRT is readily apparent to the viewer and accordingly some means for converging the three beams must be provided. Clearly the convergence units used with the older delta-type cathode ray tubes are unsuitable due to the different relative positions of the three beams.
For in-line CRT's two different approaches have been proposed. In one approach, two 4-pole and two 6-pole magnetic fields are produced, for example, by means of a toroidal magnetic core surrounding the neck of the CRT and carrying a plurality of windings: the 4-pole field is used to produce horizontal and vertical shifting of the outer beams in opposite directions while the 6-pole field is used to produce horizontal and vertical shifting of the outer beams in the same direction. With this approach there is no or little shifting of the central, usually "green", electron beam.
In the other approach, exemplified by British Patent Specification No. 1,330,827, a pair of E-shaped cores is employed, one for influencing each outer beams. A vertical magnetic field component for horizontal shifting is produced between the outer legs or pole pieces of the E-core while a horizontal magnetic field component for vertical shifting is produced between the central leg or pole piece of the E-core and the two outer pole pieces. Shielding of the central electron beam from these magnetic fields is normally required as is disclosed in British Patent Specification 1,397,804 as well as in the aforementioned specification.
The co-pending U.S. Application No. (Ser. No. 454,399), of the same filing date and title as the present application, is concerned with an E-core approach which does not require shielding of the central beam. The present invention is concerned with a convergence unit using E-cores which is simpler to manufacture and which although requiring shielding of the central beam has a high efficiency. Thus the power required to converge dynamically the beams in low making it possible to use low cost integrated-circuit drive amplifiers.